Servo brake system comprising a decompression piston integrated with the pushrod

ABSTRACT

A boosted brake ( 10 ) having a booster ( 12 ) for activating a primary piston ( 54 ) of a master cylinder ( 14 ) by way of a feeler ( 42 ), a reaction disk ( 28 ) and a push rod ( 52 ). The push rod ( 52 ) that is housed a decompression piston ( 62 ) and returned elastically by a spring ( 70 ) to a position of rest has at least two radial branches ( 64 ) in order to provide two boost ratios which differ according to the intensity of the input force.

The invention relates to a boosted brake for a motor vehicle.

The invention relates more particularly to a boosted brake for a motorvehicle, of the type which comprises a pneumatic brake booster actuatinga master cylinder, of the type in which the booster comprises a rigidcasing inside which can move a transverse partition sealably delimitinga front chamber subjected to a first pressure and a rear chambersubjected to a second pressure varying between the first pressure and apressure greater than the first pressure, of the type in which thebooster comprises a moving piston fixed to the moving partition andcomprising a front face which can act on a primary piston of the mastercylinder by way of a reaction disk housed in a cage interposed betweenthe moving piston and the primary piston, of the type in which thebooster comprises a control rod which moves in the piston selectively asa function of an axial input force exerted forward against a returnforce exerted on the rod by a return spring, of the type in which thebooster comprises a plunger which is arranged in front of the controlrod in the piston and which comprises at its rear end at least oneannular rear seat of a three-way valve which can move progressivelybetween a position in which, with the control rod at rest, the frontchamber and the rear chamber are in communication, and a position inwhich, with the control rod actuated, the second pressure prevailing inthe rear chamber increases, the valve placing the rear chamber incommunication with the pressure which is greater than the firstpressure, and of the type in which the booster comprises a feeler,arranged at the front end of the plunger and passing through a boreleading from the piston, which, when the control rod is in the restposition, is arranged at a defined jump distance from the reaction diskand which is able, when the control rod is actuated with an input forcewhose intensity is greater than a first defined intensity, to come intocontact with the reaction disk in such a way as to transmit to theplunger and to the control rod the reaction force of the mastercylinder, the ratio of the area of the reaction disk in contact with thecage to the area of the feeler in contact with the reaction diskdefining a first defined boost ratio, and of the type in which the cagecomprises at least one moving decompression wall which, when the controlrod is actuated with an input force whose intensity is greater than asecond defined intensity greater than the first, is able to move so asto create in the cage an additional volume in which a front part of thereaction disk is able to expand in order to reduce the reaction forcetransmitted to the feeler by way of the rear face of the reaction disk,the ratio of the area of the reaction disk in contact with the cage tothe area of the feeler in contact with the reaction disk defining asecond boost ratio, which is greater than the first.

Many examples of boosted brakes of this type are known.

In such a boosted brake, the moving decompression wall generally formspart of a device which is attached to the cage and which is interposedbetween the reaction disk and the primary piston of the master cylinder.

This device conventionally consists of a housing having one faceattached to the reaction disk, this face being drilled to allow thepassage of a cylindrical decompression piston which, for its part, isalso arranged in contact with the reaction disk substantially within theaxis of the feeler. The decompression piston is returned elasticallytoward the reaction disk by a helical spring which is housed inside thehousing and which has substantially the same diameter as thedecompression piston. When the intensity of the input force exceeds thesecond defined value, the decompression piston is pushed back in thehousing, compressing the helical spring, and it thus creates a freevolume which allows the reaction disk to be decompressed.

This design has many disadvantages in terms of space requirement.

Specifically, this design entails the use of a helical spring which hassubstantially the same diameter as the decompression piston. It istherefore impossible to reduce the diameter of the helical springwithout reducing the diameter of the decompression piston which, as aresult, would no longer have an appropriate surface area for creating inthe cage a decompression volume sufficient to bring about suitabledecompression of the reaction disk.

Conversely, the use of a decompression piston of suitable size does notmake it possible to use a helical spring of sufficiently small diameterfor the housing to be able, for example, to be housed in a push rodinterposed between the cage and the primary piston of the mastercylinder.

To overcome this disadvantage, the invention provides a boosted brakecomprising a decompression piston of large surface area which is able tocreate a suitable decompression volume for the reaction disk andnevertheless allows the use of a helical spring of small size.

To this end, the invention provides a boosted brake of theabove-described type, characterized in that it comprises an independentpush rod, which is interposed between the reaction disk and the primarypiston of the master cylinder, and which comprises:

-   -   a rear section having a rear face arranged in contact with the        reaction disk and in which an opening leading into the rear face        accommodates a matching decompression piston, of a defined        transverse size, which comprises at least two radial branches        and a rear face of which forms the moving decompression wall,    -   a tubular front section, of which a bore, which leads out at the        front end of the opening and has a diameter smaller than the        defined transverse size, accommodates a helical spring        accommodating a return rod of the decompression piston,        in order to provide a decompression piston of maximum transverse        size and a push rod of minimum diameter.

According to other features of the invention:

-   -   at the junction of the front section and of the rear section:        -   the rear section comprises at least one face which forms an            axial stop for the return spring of the decompression            piston, which face is arranged angularly between two            successive branches of the decompression piston and which            projects transversely inside the bore,        -   the front section comprises at least one face which forms an            axial stop for the decompression piston, which face is            arranged in the axial continuation of at least one of the            two branches of the decompression piston and which projects            inside the opening,    -   the front end of each branch of the decompression piston        comprises at least one bearing face for the helical spring,        which bearing face is arranged according to the diameter of the        tubular front section,    -   the front end of each branch of the decompression piston        comprises at least one axial stop face intended to interact with        the axial stop face of the front section,    -   the return rod of the decompression piston has a diameter        corresponding substantially to the inside diameter of the        helical spring,    -   the push rod comprises a bearing washer which is fitted between        the helical spring and the bearing face of the decompression        piston and has an outside diameter corresponding to that of the        bore of the front section and has an inside diameter        corresponding to that of the return rod of the decompression        piston,    -   the push rod comprises:        -   a tubular front part which comprises one part of the front            section,        -   a rear part which internally comprises the other part of the            front section and the rear section,    -   the front part is tightly fitted in the front-end bore of the        rear part,    -   the front part is crimped on a cylindrical front-end bearing        surface of the rear part in a variable axial position allowing        the preload of the helical spring to be adjusted,    -   the tubular front part comprises, at its front end, an axial        finger intended to allow the push rod to press on the primary        piston of the master cylinder,    -   the decompression piston comprises three branches distributed        angularly in a regular manner,    -   the decompression piston comprises two opposed branches which        extend radially as far as the periphery of the rear face of the        rear section.

Other features and advantages of the invention will become apparent onreading the detailed description which follows and which will beunderstood by referring to the appended drawing in which:

FIG. 1 is a view in axial section of a boosted brake according to aprior state of the art;

FIG. 2 is a view in axial section of a boosted brake produced accordingto the invention;

FIG. 3 is a detail view of a booster forming part of a boosted brakeaccording to the invention and comprising a first embodiment of a pushrod, represented in a position in which the control rod is actuated witha force whose intensity is smaller than the first defined intensity;

FIG. 4 is a detail view of the booster of FIG. 3 represented in aposition in which the control rod is actuated with a force whoseintensity is greater than the first defined intensity and smaller thanthe second defined intensity;

FIG. 5 is a detail view of the booster of FIG. 3 represented in aposition in which the control rod is actuated with a force whoseintensity is greater than the second defined intensity;

FIG. 6 is an assembled perspective view of a variant of the firstembodiment of the push rod of the booster of FIGS. 3 to 5;

FIG. 7A is an exploded perspective view of the front of the rear part ofthe push rod of FIGS. 3 to 5;

FIG. 7B is an exploded perspective view of the rear of the rear part ofthe push rod of FIGS. 3 to 5;

FIG. 8 is an assembled perspective view of a second embodiment of thepush rod of the booster;

FIG. 9A is an exploded perspective view of the front of the rear part ofthe push rod of FIG. 8;

FIG. 9B is an exploded perspective view of the rear of the rear part ofthe push rod of FIG. 8.

In the description which will follow, identical reference numbers denotecomponents which are identical or have similar functions.

By convention, the terms “front” “rear” “upper” and “lower” respectivelydenote elements or positions which are respectively directed toward theleft, the right, the top or the bottom of the figures.

FIG. 1 shows the complete boosted brake 10 for a motor vehicle.

In a known way, the boosted brake 10 comprises a pneumatic brake booster12 which is intended to activate a master cylinder 14.

The booster 12 comprises a rigid casing 16 inside which can move atransverse partition 18 sealably delimiting a front chamber 20 subjectedto a first pressure “P₁” and a rear chamber 22 subjected to a secondpressure “P₂” varying between the first pressure “P₁” and a pressure“P_(a)” which is greater than the first pressure “P₁”.

In a known way, the booster 12 comprises a moving piston 24 fixed to themoving partition 18 and comprising a front face 26 which is able to acton a primary piston of the master cylinder 14 by way of a reaction disk28. The reaction disk 28 is housed in a cage 30 which is interposedbetween the moving piston 24 and the primary piston (not shown).

The booster 12 also comprises a control rod 32 which moves in the piston24 selectively as a function of an axial input force exerted toward thefront against a return force exerted on the rod 32 by a return spring34.

The booster 12 comprises a plunger 36 which is arranged in front of thecontrol rod 32 in the piston 24 and which comprises at its rear end 38at least one annular rear seat of a three-way valve 40 which can moveprogressively between a position in which, with the control rod 32 atrest, the front chamber 20 and the rear chamber 22 are in communication,and a position in which, with the control rod 32 actuated, the secondpressure “P₂” prevailing in the rear chamber 22 increases, the valve 40placing the rear chamber 22 in communication with the pressure “P_(a)”which is greater than the first pressure.

The booster 12 comprises a feeler 42, arranged at the front end of theplunger 36 and passing through a bore 44 leading from the piston 24,which, when the control rod 32 is in the rest position, is arranged at adefined jump distance from the reaction disk 28 and which is able, whenthe control rod 32 is actuated with an input force whose intensity isgreater than a first defined intensity, to cross over the jump distanceand come into contact with the reaction disk 28 in such a way as totransmit the reaction force of the master cylinder 14 to the plunger 36and to the control rod 32.

In this configuration, the ratio of the area of the reaction disk 28 incontact with the cage 30 to the area of the feeler 42 in contact withthe reaction disk 28 in a known way defines a first defined boost ratio.

The disadvantage of this known design is that when the control rod isactuated rapidly with an input force whose intensity is greater than thefirst defined intensity, the delay in the balancing of the pressuresbetween the front chamber 20 and rear chamber 22 does not allow themoving piston 24 to sufficiently boost the driver's braking force, whichmust, moreover, overcome the reaction force that the master cylinder 14exerts on the rod 32 by way of the reaction disk 28.

To overcome this disadvantage, a boosted brake 10 (not shown) of thetype described above has been provided in which the cage 30 comprises atleast one moving decompression wall which, when the control rod 32 isactuated with an input force whose intensity is greater than a seconddefined intensity greater than the first, is able to move so as tocreate in the cage 30 an additional volume in which a front part of thereaction disk 28 is able to expand with respect to the feeler 42.

In this way, a rear part of the reaction disk 28 is able to decompresslocally at the level of the feeler 42, which makes it possible to reducethe reaction force transmitted to the feeler 42 by way of the rear faceof the reaction disk.

Moreover, in this configuration, since the area of the feeler 42 incontact with the reaction disk is small, the defined boost ratioincreases. This second boost ratio is therefore greater than the first.

In such a boosted brake, the moving decompression wall generally formspart of a decompression device which is attached to the cage and whichis interposed between the reaction disk and the primary piston of themaster cylinder.

This boosted brake (not shown) conventionally comprises a housing fixedto the primary piston of the master cylinder, one face of which housingis attached to the reaction disk and is drilled to allow the passage ofa cylindrical decompression piston which, for its part, is also arrangedin contact with the reaction disk. The decompression piston is returnedelastically toward the reaction disk by a helical spring which is housedinside the housing and which has substantially the same diameter as thedecompression piston. When the intensity of the input force exceeds thesecond defined value, the decompression piston is pushed back in thehousing, compressing the helical spring, and it thus creates a freevolume which allows the reaction disk to be decompressed.

This design has many disadvantages in terms of space requirement.

Specifically, this design entails the use of a helical spring which hassubstantially the same diameter as the decompression piston. It istherefore impossible to reduce the diameter of the helical springwithout reducing the diameter of the decompression piston which, as aresult, would no longer have an appropriate surface area for creating inthe cage a decompression volume sufficient to bring about suitabledecompression of the reaction disk.

Conversely, the use of a decompression piston of suitable size does notmake it possible to use a helical spring of sufficiently small diameterfor the housing to be able, for example, to be integrated in a push rodinterposed between the cage and the primary piston of the mastercylinder.

To overcome this disadvantage, as illustrated in FIG. 2 and thesubsequent figures, the invention provides a boosted brake 10 of theabove-described type comprising a compact decompression device.

In a known way, the boosted brake 10 is provided with a cage 30, formedin the piston 24, at least one moving decompression wall 46 of whichcage is able, when the control rod 32 is actuated with an input forcewhose intensity is greater than a second defined intensity greater thanthe first, to move so as to create in the cage 30 an additional volumein which a front part 48 of the reaction disk 28 is able to expand inorder to reduce the reaction force transmitted to the feeler 42 by wayof the rear face 50 of the reaction disk 28.

More particularly, according to the invention and as illustrated in FIG.2 and the subsequent figures, the booster 12 comprises an independentpush rod 52, which is interposed between the reaction disk 28 and theprimary piston 54 of the master cylinder 14, and which comprises a rearsection 56 having a rear face 58 intended to be arranged in contact withthe reaction disk 28 and in which an opening 60 leading into the rearface 58 accommodates a matching decompression piston 62, of a definedtransverse size “E”, which comprises at least two radial branches 64 anda rear face of which forms the moving decompression wall 46.

FIGS. 7A and 7B represent a first embodiment of a piston 62 comprisingthree radial branches 64 distributed angularly in a regular manner, andFIGS. 9A and 9B represent a second embodiment of the inventioncomprising two radial branches 64 which extend radially as far as theperiphery of the rear face 58 of the rear section 56.

It will be understood that these arrangements in no way restrict theinvention and that the piston 62 could comprise a greater number ofbranches 64, for example 4, 5 or 6 radial branches 64.

The independent push rod 52 also comprises a tubular front section 66,of which a bore 68, which leads out at the front end of the opening 60and has a diameter “D” smaller than the defined transverse size “E”accommodates a helical spring 70 accommodating a return rod 72 of thedecompression piston 62. Preferably, the return rod 72 of thedecompression piston has a diameter corresponding substantially to theinside diameter of the helical spring 70.

In this way, the boosted brake 10 provides a decompression piston 62 ofmaximum transverse size “E” and a push rod 52 of minimum diameter,substantially close to the diameter “D” of its bore 68.

It will be understood that the front section 66 and rear section 56 areassociated with the bore 68 and the opening 60, respectively, and thatthey are not necessarily independent. Thus, in the preferred embodimentsof the invention, the push rod 52 comprises a tubular front part 67which comprises one part of the front section 66 and a rear part 57which internally comprises the other part of the front section 66, andtherefore the other part of the bore 68, and the rear section 56.

More specifically, as illustrated in FIGS. 7A and 9A, at the junction ofthe front section 66 and of the rear section 56, the rear section 56comprises at least one face 74 which forms an axial stop for the returnspring 70 of the decompression piston 62.

The face 74, arranged at the end of the opening 60, is arrangedangularly between two successive branches 64 of the decompression piston62 and it projects transversely inside the bore 68. Thus, the rear part57 associated with the piston 62 having three branches 64 as shown inFIGS. 3 to 7B comprises three faces 74 which are distributed between thethree branches of the opening 60 corresponding to the three branches 64of the piston 62, while the rear part 57 associated with the piston 62having two branches 64 as shown in FIGS. 8 to 9B comprises two faces 74distributed between the two branches of the opening 60 corresponding tothe two branches 64 of the piston 62.

Moreover, as illustrated in FIGS. 3 to 6, 7B, 8 and 9B, the frontsection 66 comprises, in the rear part 57, at least one face 76 whichforms an axial stop for the decompression piston 62. The face 76 isarranged in the axial continuation of at least one of the two branches64 of the decompression piston 62 and it projects inside the opening 60.Each rear part 57 therefore comprises as many stop faces 76 as thepiston 62 comprises branches 64.

As illustrated in FIGS. 7A and 9A, to allow the piston 62 to returnwhile retaining a piston 62 which has simple shapes and is axiallycompact, the front end of each branch 64 of the decompression piston 62comprises at least one bearing face 78 for the helical spring 70, whichbearing face is arranged according to the diameter of the tubular frontsection 66.

In the configurations described with reference to the first and secondembodiments of the invention, the faces 78 are distributed over all thebranches 64 of the piston 62. The piston 62 of FIG. 7A thus comprisesthree faces 78 and the piston 62 of FIG. 9A comprises a face 78 shapedin the form of an annular shoulder face.

The spring 70 may bear directly on the face 78, but the push rod 52preferably comprises a bearing washer 79 which is fitted between thehelical spring 70 and the bearing face 78 of the decompression piston62. This washer 79 has an outside diameter corresponding to that of thebore 68 of the front section and it has an inside diameter correspondingto that of the return rod 72 of the decompression piston 62.

Thus, when the piston 62 occupies its rear position as shown in FIGS. 3,4, 6 and 8, the spring 70 bears in the same plane on the face 78 of thepiston and the face 74 of the rear section 56, which makes it possibleto benefit from a particularly axially compact rear part 57.

In this way, the moving wall 46 can move as represented in FIGS. 3 to 5.

In an initial position represented in FIG. 3, a zero input force, or aforce whose intensity is smaller than a first defined intensity, isapplied to the control rod 32. In this configuration, the feeler 42 doesnot act on the reaction disk 28 and it is arranged at a jump distance“d” from the latter.

In an intermediate position represented in FIG. 4, an input force isapplied to the control rod 32 with an input force whose intensity isgreater than the first defined intensity and smaller than a seconddefined intensity. In this configuration, the reaction disk 28 isdeformed and substantially closes up the jump distance “d” separating itfrom the feeler 42 in order to transmit the input force to the primarypiston 54 according to the first defined boost ratio. In thisconfiguration, since the input force is smaller than the preload of thespring 70, the decompression piston 62 is not acted upon and remains inalignment with the rear face 58 of the section 56. The reaction disk 28transmits the whole of the reaction of the master cylinder 14 to thefeeler 42.

In the decompression position represented in FIG. 5, an input force isapplied to the control rod 32 with an input force whose intensity isgreater than the second defined intensity. In this configuration, thefeeler 42 acts on the reaction disk 28, which overcomes the antagonisticforce of the spring 70, and the decompression piston 62 pushes into theopening 60, locally decompressing the reaction disk 28 in contact withthe feeler 42. The input force is transmitted to the primary piston 54according to the second defined boost ratio, which is greater than thefirst. The reaction disk 28 no longer transmits more than a fraction ofthe reaction of the master cylinder 14 to the feeler 42.

Advantageously, the front end of each branch 64 of the decompressionpiston also comprises at least one axial stop face 80 intended tointeract with the axial stop face 76 of the front section 66.

In the first and second embodiments represented in FIGS. 3 to 5 and 8,the front part 67 is crimped on a cylindrical front-end bearing surface82 of the rear part 57 in a variable axial position allowing the preloadof the helical spring 70 to be adjusted. For example, a key 84 may beinterposed between the parts 67 and 57.

As an alternative, shown in FIG. 6, the front part could be tightlyfitted in a front end bore 88 of the rear part 57. In that case, thepreload of the spring 70 is defined by the depth of the fit, which canbe limited by a key 86 housed at the bottom end of the bore 88.

Finally, it should be pointed out that the invention takes advantage ofthe tubular design of the front part. As illustrated in FIGS. 3 to 6 and8, the tubular front part 67 accommodates, at its front end, an axialfinger 90 which is, for example, crimped in a matching dish-shapedbearing surface 92 at the front end of the front part 67 and which isintended to allow the push rod 52 to press into a bore 91 of the primarypiston 54. Advantageously, the dish-shaped bearing surface 92 defines,in the front part, a dish 94 allowing the spring 70 to bear.

1. A boosted brake (10) for a motor vehicle comprising a pneumatic brakebooster (12) for actuating a master cylinder (14), said booster (12)comprises a rigid casing (16) inside which moves a transverse partition(18) sealably delimiting a front chamber (20) subjected to a firstpressure (P₁) and a rear chamber (22) subjected to a second pressure(P₂) varying between the first pressure (P₁) and a pressure (P_(a))greater than the first pressure (P₁), a moving piston (24) fixed to themoving partition (18) and comprising a front face (26) which acts on aprimary piston (54) of the master cylinder (14) by way of a reactiondisk (28) housed in a cage (30) interposed between the moving piston(24) and the primary piston (54), a control rod (32) which moves in thepiston (24) selectively as a function of a first axial input forceexerted forward against a return force exerted on the rod (32) by areturn spring (34), a plunger (36) which is arranged in front of thecontrol rod (32) in the piston (24) and a rear end (38) with at leastone annular rear seat of a three-way valve (40) which progressivelymoves between a position in which, with the control rod (32) at rest,the front chamber (20) and the rear chamber (22) are in communication,and a position in which, with the control rod (32) actuated, the secondpressure (P₂) prevailing in the rear chamber (22) increases, the valve(40) placing the rear chamber (22) in communication with the pressure(P_(a)) which is greater than the first pressure (P₁), and a feeler(42), arranged at the front end of the plunger (36) and passing througha bore (44) leading from the piston (24), which, when the control rod(32) is in the rest position, is arranged at a defined jump distance (d)from the reaction disk (28) and which, when the control rod (32) isactuated with an input force whose intensity is greater than a firstdefined intensity, comes into contact with the reaction disk (28) insuch a way as to transmit to the plunger (36) and to the control rod(32) a reaction force of the master cylinder (14), the ratio of the areaof the reaction disk (28) in contact with the cage (30) to the area ofthe feeler (42) in contact with the reaction disk (28) defining a firstdefined boost ratio, and the said cage (30) comprises at least onemoving decompression wall (46) which, when the control rod (32) isactuated with a second axial input force whose intensity is greater thana second defined intensity greater than said first axial input force,moves to create in the cage (30) an additional volume in which a frontpart (48) of the reaction disk (28) expands in order to reduce thereaction force transmitted to the feeler (42) by way of the rear face(50) of the reaction disk (28), the ratio of the area of the reactiondisk (28) in contact with the cage (30) to the area of the feeler (42)in contact with the reaction disk (28) defining a second boost ratio,which is greater than the first boost ratio, characterized in that saidcontrol rod (32) comprises an independent push rod (52) which isinterposed between the reaction disk (28) and the primary piston (54) ofthe master cylinder, and which comprises: a rear section (56) having arear face (58) arranged in contact with the reaction disk (28) and inwhich an opening (60) leading into the rear face (58) accommodates amatching decompression piston (62), of a defined transverse size (E),which comprises at least two radial branches (64) and a rear face ofwhich forms the moving decompression wall (46), a tubular front section(66) of the outside diameter of the primary piston (54) of the mastercylinder (14), of which a bore (68), which leads out at the front end ofthe opening (60) and has a diameter (D) smaller than the definedtransverse size (E), accommodates a helical spring (70) accommodating areturn rod (72) of the decompression piston (62), in order to provide adecompression piston (62) of maximum transverse size (E) and a push rod(52) of minimum diameter.
 2. The boosted brake (10) according to claim1, characterized in that, at the junction of the front section (66) andof the rear section (56): the rear section (56) comprises at least oneface (74) which forms an axial stop for the return spring (70) of thedecompression piston (62), which face is arranged angularly between twosuccessive branches (64) of the decompression piston (62) and whichprojects transversely inside the bore (68); and the front section (66)comprises at least one face (76) which forms an axial stop for thedecompression piston (62), which face is arranged in the axialcontinuation of at least one of the two branches (64) of thedecompression piston (62) and which projects inside the opening (60). 3.A boosted brake (10) according to claim 2, characterized in that a frontend of each branch (64) of the decompression piston (62) comprises atleast one bearing face (78) for the helical spring (70), which bearingface is arranged according to the diameter (D) of the tubular frontsection (66).
 4. A boosted brake (10) according to claim 3,characterized in that the front end of each branch (64) of thedecompression piston comprises at least one axial stop face (80) thatinteracts with the axial stop face (76) of the front section (66).
 5. Aboosted brake (10) according to claim 4, characterized in that thereturn rod (72) of the decompression piston (62) has a diametercorresponding substantially to the inside diameter of the helical spring(70).
 6. A boosted brake (10) according to claim 5, characterized inthat the push rod (52) comprises a bearing washer (79) which is fittedbetween the helical spring (70) and the bearing face (78) of thedecompression piston (62) and has an outside diameter corresponding tothat of the bore (68) of the front section (66) and has an insidediameter corresponding to that of the return rod (72) of thedecompression piston (62).
 7. A boosted brake (10) according to claim 6,characterized in that the push rod (52) comprises: a tubular front part(67) which comprises one part of the front section (66); and a rear part(57) which internally comprises the other part of the front section (66)and the rear section (56).
 8. A boosted brake (10) according to claim 7,characterized in that the front part (67) is tightly fitted in afront-end bore (88) of the rear part (57).
 9. A boosted brake (10)according to claim 7, characterized in that the front part (67) iscrimped onto a cylindrical front-end bearing surface (82) of the rearpart (57) in a variable axial position allowing the preload of thehelical spring (70) to be adjusted.
 10. A boosted brake (10) accordingto claim 9, characterized in that the tubular front part (67) comprises,at its front end, an axial finger (90) intended to allow the push rod(52) to press on the primary piston (54) of the master cylinder (14).11. A boosted brake (10) according to claim 10, characterized in thatthe decompression piston (62) comprises three branches (64) distributedangularly in a regular manner.
 12. A boosted brake (10) according toclaim 10, characterized in that the decompression piston (62) comprisestwo opposed branches (64) which extend radially as far as the peripheryof the rear face (58) of the rear section (56).